The invention relates to systems for drilling into the surface of the earth, and in particular to such systems incorporating means to measure and guide the bit to the desired direction.
In nuclear weapons testing, the mining industry, and the oil industry, there is a need for guided drilling or coring. In the case of the mining and oil industries, a subsurface location containing a desirable resource sometimes needed to be reached by a hole that is as straight as possible (McCray, Arthur and Cole, Frank, Oil Well Drilling Technology, Norman: Univ. of Oklahoma Press, 1967; 306). A "straight" hole in this case is not only the shortest distance from the surface to the location of interest, but also is parallel to the local gravity vector. FIG. 1 illustrates what happens when a bit deviates from an ideally straight path. The bit starts at point A, and instead of going to the ideal point B, it ends at point C. The deviated point C defines the error in terms of the deviation angle .alpha., or the distance from the ideal endpoint .delta..
Hole deviation is caused by non-uniform formations and inclined rock strata, and in the case of a conventional bit, can be accelerated by the bending of the drill string. With a conventional bit, a hole is usually considered straight if the deviation angle .alpha. is less than 5 degrees.
In nuclear weapons testing the CORRTEX method is proposed to verify the strength of nuclear bomb blasts. This method involves drilling a straight hole near the bomb and inserting a tube or "core" in the hole. When the blast occurs, the core is crushed at a rate indicative of the blast strength (Norman C. "Test Ban Talks Reach Impasse (Seismic Detection vs. CORRTEX)." Science April 15, 1988; 273). The "straightness" requirement for this kind or hole is a deviation distance .delta. of 2 feet in a 2000 foot deep hole. The hole deviation angle .alpha. in this case is only 0.057 degrees.
Precise vertical drilling, in general, requires slow drilling rates so that the drilling assembly seeks the vertical. During a conventional vertical drilling, the drill steel is stopped in order to measure the orientation of the hole and determine the proper correction, only then the drilling is resumed. Some conventional steering methods require placing a metal wedge on the bottom of the hole to deflect the drill to the desired direction, or using a hydraulic piston pushing against the wall of the hole to deflect and re-orient the drill. Thus, this type of straight-hole vertical drilling is slow and also dependent on the geological conditions.
In summary, there is a need for a continuously guided drilling system which can operate at conventional drilling rates of tens of feet per hour or higher.